1. Field of the invention
The present invention relates to the chemical decomposition of organic molecules, and, more particularly, to chemical decomposition through the use of sonication in liquid carbon dioxide.
2. Description of Related Art
Ultrasonic cleaning has been utilized by industry for a number of years. In the conventional processes, the sonicating media are organic solvents, or water and aqueous solutions, and ultrasonic energy is applied to the media to promote cavitation, i.e., the formation of "vacuum" bubbles and their subsequent collapse. Although adequate for the sonication cleaning, both of these approaches has a significant disadvantage. The use of organic and aqueous solvents as sonicating media present the problem of chemical disposal of the spent sonication medium and is subject to strict regulatory controls. An additional disadvantage relates to handling of the removed contaminant(s), whether organic or particulate. When the contaminant is a controlled material, once in solution or suspension, its volume is substantially increased, and this presents an additional post-treatment/disposal problem.
In these conventional ultrasonic cleaning processes, sonic horns are often used to produce the sonic energy. In other processes, a cavitation nozzle may be used. For example, U.S. Pat. No. 4,906,387, issued Mar. 6, 1990, to J. Pisani for "Method for Removing Oxidizable Contaminants in Cooling Water Used in Conjunction with a Cooling Tower" and U.S. Pat. No. 4,990,260, issued Feb. 5, 1991, to J. Pisani for "Method and Apparatus for Removing Oxidizable Contaminants in Water to Achieve High Purity Water for Industrial Use" disclose methods for removing contaminants from water by inducing cavitation in the water to cause the water to dissociate to produce hydroxyl free-radicals which act as oxidizing agents. In the processes of Pisani, ultraviolet (UV) radiation is used in combination with cavitation to continue the oxidation process which was initiated by the hydroxyl free-radicals. The cavitation in the Pisani processes is produced by a "critical flow" nozzle.
Another type of cleaning process, utilizing phase shifting of dense phase gases, has been disclosed and claimed in U.S. Pat. No. 5,013,366, issued to D. P. Jackson et al and assigned to the same assignee as the present application. The process employs a dense phase gas at or above the critical pressure. The phase of the dense phase gas is then shifted between the liquid state and the supercritical state by varying the temperature of the dense fluid in a series of steps between temperatures above and below the critical temperature of the dense fluid, while maintaining the pressure above the critical value. Examples of fluids include (1) hydrocarbons, such as methane, ethane, propane, butane, pentans, hexane, ethylene, and propylene; (2) halogenated hydrocarbons, such as tetrafluoromethane, chlorodifluoromethane, and perfluoropropane; (3) inorganics, such as carbon dioxide, ammonia, helium, krypton, argon, sulfur hexafluoride, and nitrous oxide; and (4) mixtures thereof. In alternative embodiments, the dense phase gas may be exposed to UV radiation during the cleaning process or ultrasonic energy may be applied during the cleaning process to agitate the dense phase gas and the substrate surface. These treatments enhance removal efficiency of the contaminant from the surface being cleaned, but still result in a waste stream requiring further treatment and/or disposal.
More recently, in view of environmental concerns about the contaminating effect of certain organic molecules, investigations have begun to determine ways to deal with such organics. In one approach, destruction of organics such as polychlorobenzenes (PCBs) through irradiation with 20 Kilohertz (Khz) sonic energy has been recently documented, using nonaqueous solvents as extracting/sonicating medium. Ultrasonic energy has also been used in conjunction with hydrogen peroxide to treat gasoline/benzene contaminated ground water. During cavitation, the energy of the imploding bubbles initiate the decomposition reactions.
The use of solvents (aqueous or organic) as sonicating media presents the problem of the sonicating media disposal/regeneration and the disposal of the reaction products themselves. Accordingly, there is a need to provide a method of destroying organic molecules, employing a sonicating medium that presents no environmental disposal problems.